Frequency conversion circuits



y 3. 1939 R. L, WLLER 2,159,591

FREQUENCY CONVERSION CIRCUITS Original Filed July 31, 1937 FIG/ FIG. IAmsousucr MUL T/PL IER NON-LINEAR TUNED o IMPED 3 F/G 3 2/3 TUNED TOINVENTOR RL. M/L L E R arf:

A T TORNE V Patented May 23, 1939 UNITED STATES PATENT OFFICE FREQUENCYCONVERSION CIRCUITS Original application July 31, 1937, Serial No.156,698. Divided and this application April 30, 1938, Serial No. 205,225

6 Claims.

This application is a division of my copending application, Serial No.156,698, filed July 31, 1937. The invention relates to the production ofalternating current waves by frequency conversicn methods.

An object of the invention is to convert alternating current waves ofone frequency into alternating current waves of a different frequency.

A more specific object is to produce alternating l current waves ofdesired frequencies which are accurate fractions of a given basefrequency.

These objects are attained in accordance with the invention by frequencyconversion circuits utilizing a process which may be termed regen- 15erative modulation. Regenerative modulation is produced in general byfeeding back the output of a balanced type modulator to the balanced orconjugate input thereof through a selective network, such as a. filter,and an amplifier of fixed gain mu. Such a circuit is stable and of anonoscillatory nature as long as the loss due to the balanced conditionand the network or filter is greater than the gain mu of the amplifier.

The frequency conversion circuits of the invention employing thisprocess, to be described hereinafter, may be used to produce electricalwaves which are exact fractional ratios of a given frequency applied tothe input, and which will follow amplitude and frequency variations inthe :30 applied waves over quite wide ranges, these circuits havingexceptional frequency stability and efficiency in operation.

The various features and advantages of the circuits of the inventionwill be brought out in the following detailed description thereof whenread in connection with the accompanying drawing of which:

Fig. 1 shows schematically a circuit which illustrates the basicprinciples of the invention;

Fig. 1A shows a modification of this basic circuit; and

Figs. 2 and 3 show schematically in greater detail frequency conversioncircuits embodying different modifications of the invention.

The fundamental concept of regenerative modulation as applied tofrequency conversion may be described by referring to Fig. 1.

The frequency converter circuit of Fig. 1 includes a balanced modulatorl of the second order type, such as that disclosed in F. A. Cowan PatentNo. 1,959,459, consisting of four copperoxide rectifier units connectedin a Wheatstone bridge formation, an input transformer 2 and an outputtransformer 3, the secondary winding of the input transformer and theprimary winding of the output transformer being connected in shunt withone diagonal 4, of the bridge and a source of modulating current beingconnected to the other conjugate diagonal 6, I of the bridge. Thecopper-oxide rectifier units 8, 9 are poled so that each is conductivein the direction toward the common terminal 6, and the other rectifierunits [0, II are poled so that each is conductive away from the commonterminal I. The primary winding of the input transformer 2 is connectedto a source (not shown) of the base frequency ii to be converted, and afilter I2 is connected between the secondary winding of the outputtransformer 3 and the output terminal of the circuit. The source ofmodulating waves applied to the conjugate terminals 6, l of themodulator bridge is the regenerative circuit I3 comprising the filternetwork 12 and the one-way amplifier M of gain mu having its inputconnected across the output of filter l2 and its output connected acrossthe conjugate terminals 6, 'l of the bridge I through the transformerl5. The gain mu of the amplifier [4 in the regenerative circuit isselected so as to provide the required stability. As indicated, afrequency multiplier l6, indicated by the box so labeled in Fig. 1A,would be included in the regenerative circuit [3 where it is desired tosecure a fractional frequency having a denominator larger than 2.

Now if the input frequency I1 is applied to the input of the secondorder modulator in the system of Fig. l and any frequency f2 in theoutput thereof is selected by the filter I2 and fed back through theamplifier l4 and transformer l5 to the conjugate terminals 6, l of themodulator, the two frequencies f1 and f2 will combine in the modulatorto produce the two side-band frequencies hi which are at a certain losswith respect to f2. If the amplification mu is greater than theside-band loss plus the loss provided by the filter l2, the side-bandfrequencies will be fed back and will be applied to the conjugateterminals 8, 9 of the modulator I but at a greater amplitude. If thefollowing arbitrary case is set up which is the case where f2 willsustain itself, it will be found that so that the wave appearing in theoutput of the frequency conversion circuit is half the frequency of thewave applied to the input thereof from the base frequency source, or

ii 2 as indicated.

In the general case for this type of circuit depending upon the order ofmodulation used nfi i m z z or n l i m 1 Where n and m are integersdepending upon the order of modulation. For the case of third ordermodulation, where a third order instead of a second order modulator isused,

11 1, rn=2, or 11 2, 122 1 and In the case where the frequencymultiplier I5 of Fig. 1A is used in the regenerative circuit I3, thefollowing equations may be set up:

Where is is the frequency in the output of the multiplier and 1' is thefactor by which the feedback frequency f2 is multiplied.

Solving these two equations simultaneously gives .Tl 2 i In f f (10,

If the input wave is represented by the equation and the frequencycomponent f2 at the input of the modulator is represented as e =B costhen the two side-band outputs which are obtained are given by where 6is the phase shift which may be introduced by the filter I2 and theamplifier I4. From this is obtained and it has been found that thefrequency Jz in the feedback circuit will automatically adjust its phasewith respect to f1, so that its phase is right to reproduce itself, andits ability to produce a sustained frequency should be independent ofany phase shift in the feedback circuit. This action has been verifiedexperimentally.

An important consideration is the relation of the amplitude of asub-harmonic component to that of the fundamental. Since thesub-harmonic is produced by a process of modulation its amplitude cannotexceed that of the fundamental. Thus, there can be no runaway conditionas in an ordinary oscillation which is limited only by the overloadcapacity of the oscillator. The amplitude of the sub-harmonic will besuch that the modulation loss is equal to the gain mu of the amplifier,less the loss of the filter net work. In general, the amplitude of thesub-harmonic will be a function of the fundamental, although it may notbe a linear relation.

Figs. 2 and 3 show frequency conversion circuits in accordance with theinvention employing third order regenerative modulators instead of asecond order modulator as in the circuit of the previous figure.

The circuit of Fig. 2 may be employed for reducing the base frequency fto the sub-multiple frequency The modulator portion of the circuitcomprises the two non-linear modulating elements I84 and I85 eachcomprising a plate of a material of the sort hereinafter described, heldbetween a pair of terminal electrodes, serially connected between aninput transformer I86 and an output transformer I81. The primary windingof the input transformer I86 is connected to the source of basefrequency f (not shown) to be converted. The control grid-cathodecircuit of a three-electrode space discharge amplifying tube I88 isconnected across the secondary winding of the transformer I8? andincludes in series with that winding the usual grid-biasing arrangementcomprising a resistance I89 shunted by a condenser I SI]. A condenserISI connected across the secondary winding of transformer I81 forms withthe inductance thereof an anti-resonant circuit. The primary winding ofa combined output and feedback transformer I92 is connected in theanode-cathode circuit of tube I88 in series with the plate battery I93.The heater type cathode of tube I88 is supplied with heating currentfrom the battery I94 as shown. The usual condenser I95 for by-passingthe a-c component of the output current from the plate battery isconnected from the lower terminal of the primary winding of transformerI92 to the cathode of the tube I88.

The feedback circuit I96 has its input connected across the terminals ofthe secondary winding of transformer I92 and its output terminalsconnected across the mid-point of the secondary winding of the modulatorinput transformer I85 and the mid-point of the primary Winding of themodulator output transformer I37. The terminals of the secondary windingof transformer I92 are also connected to a utilization circuit for theconverted sub-multiple frequency The plate of the non-linear modulatingelements I84 and I85 in the modulator circuit of Fig. 2 preferably is ofa material which comprises a mass of finely divided conductive orsemi-conductive crystalline particles held together in random contact ina binding matrix of insulating substance. One example of a suitablematerial of this kind is a mixture of silicon carbide and carbon withclay or kaolin as a binder, as disclosed in the U. S. Patent 1,822,742issued September 8, 1931 to K. B. McEachron.

In the operation of the system of Fig. 2, the base frequency isimpressed on the modulating elements I84 and I85 by the inputtransformer .186 of this third order modulator, and of the modulationproducts appearing in the output of the modulating elements thesub-multiple frequency is selected by the anti-resonant circuitcomprising the secondary winding of output transformer I81 and the shuntcondenser I9I, and will be amplified by the amplifying device I88. Theamplified sub-multiple appearing in the secondary winding of thetransformer I92 will be fed back to the conjugate balanced input of themodulator by the feedback circuit I96 and will combine in the modulatorwith the base frequency f applied through input transformer I86 toproduce, as indicated by Equation 6 above, the combination frequenciesThe frequency f will be eliminated by the reso nant circuit comprisingthe secondary winding of transformer I81 and the shunt condenser I9] andthe sustained sub-multiple frequency selected thereby will be amplifiedby the amplifier I88 and the amplified wave of the frequency impressedby transformer I92 on the utilization circut as indicated.

As in the other circuit described, the circuit constants of the tunedamplifier I88 should be adjusted to make the gain of the feedbackcircuit greater than the loss of the modulator and resonant network togive the required stability in operation.

The circuit of Fig. 3 is suitable for reducing a base frequency f to anexact sub-multiple thereof. As shown, the modulator and the succeedingamplifier in the circuit of Fig. 3 are identical with the third ordermodulator and the amplifier used in the system of Fig. 2, as indicatedby the use of the same characters for identifying their circuitelements. However, the transformer I91 connected to the output of theamplifying tube I88, in addition to a primary winding I98 in series inthe cathode-anode circuit of the tube I88, and a secondary winding I99connected to the utilization circuit, has a third winding 200 coupled tothe winding I98, which is connected to the input of the doublediode-triode tube 20I.

The double diode rectifier portion of the tube 20I is utilized as afrequency multiplier, and the nected to the mid-point of the winding 200of transformer I91 through the resonant circuit 204 and the resistance205 in series, and the two rectifier anodes 206 and 201 respectivelyconnected directly to the terminals of the winding 200.

The amplifier portion of the tube 20I comprises the cathode 203, thecontrol grid 208 and the plate or anode 209. The control grid 208 oftube 20I is connected to the cathode 203 through the resistance 2 l0,and to the mid-point of winding 200 of transformer I91 through thecondenser 2| I and resistance 205 in series.

The anode-cathode circuit of the amplifier portion of the tube 20Iincludes in series the primary winding of the feedback transformer 2I2,the secondary winding of which is connected across the mid-point of thesecondary Winding of the modulator input transformer I88 and themid-point of the primary winding of the modulator output transformerI81, as indicated. Space current is supplied from plate battery 2 I3 tothe anode of amplifier tube I88 through the primary winding I98 oftransformer I91, and to the anode of the amplifier portion of the tube20I through the primary winding of feedback transformer 2I2. Heatingcurrent is supplied from battery 2 I4 to the heaters for the cathodes oftubes I88 and 20I in series through series resistance 2I5.

The circuit of Fig. 3 operates as follows: The wave of the basefrequency f is impressed upon the modulator circuit by the inputtransformer I86 and a desired sub-multiple frequency in the modulatoroutput is selected by the anti-resonant circuit comprising the secondarywinding of transformer I81 and shunt condenser I 9|. This sub-multiplefrequency is amplified by the tube I88 and the amplified sub-multiplefrequency appearing in the winding I98 of transformer I91 is induced inthe winding 200 coupled thereto,forming the input coil for the doublediode rectifier portion of the tube 20I, acting as a frequencymultiplier. The desired frequency multiple which is determined by thetuning of the resonant circuit 204 is impressed through condenser 2 IIupon the grid circuit of the amplifier portion of the tube 20I and isamplified thereby. The amplified multiple frequency is impressed byfeedback transformer 2 I2 on the conjugate balanced input of themodulator and modulates therein with the wave of base frequency fimpressed on the input of the modulator through input transformer I88.The desired combination frequency in the output of the modulator isselected by the anti-resonant circuit comprising the secondary windingof transformer I81 and the shunt condenser I9I. The selected "componentis amplified by the amplifier I88 and impressed through the coils I98and I99 of transformer I91 on the utilization circuit.

The sub-multiple at which the frequency conversion circuit of Fig. 3operates depends upon the tuning of the two amplifiers therein, that is,the amplifier I88 and the amplifier portion of the double diode-triodetube 20I. The tuning of the first amplifier is determined by theresonant frequency of the anti-resonant circuit in the input thereofcomprising secondary winding of transformer I81 and the shunt condenserI9I, and the tuning of the second amplifier is determined by theresonant frequency of the resonant circuit 284. For the case in which itis desired to obtain the sub-multiple frequency the first resonantcircuit would be tuned to the frequency and the resonant circuit 284would be tuned to so that the harmonic will be fed back into theconjugate input of the modulator by feedback transformer M2. Thefrequency will combine in the modulator with the input frequency f toproduce the combination frequencies The former frequency will beeliminated the tuned input of amplifier 588 and the latter fre quency,

will be selected thereby, amplified by the amplifier E38 and impressedby transformer I 9? on the utilization circuit connected to the windingI99 thereof.

Other types of non-linear elements which will provide third ordermodulation may be substituted for the particular modulating elements I84and 385 described for the modulators in the systems of Figs, 2 and 3,for example, magnetic devices, such as saturable core coils ortransformers well known in the art.

Other modifications of the circuits illustrated and described abovewithin spirit and scope of the invention will occur to persons skilledin the art.

What is claimed is:

1. A frequency converter comprising a balanced third order modulatorhavi ,5 two ccnjugately connected inputs, means for impressing on one ofsaid inputs a Wave of a given frequency to be converted to a wave ofanother frequency wl'zich is a desired fraction of said giv n frequency.a circuit including a tuned amplifier coupling the output of saidmodulator to the other input thereof, designed to selectively feed backto said other input waves derived from the output of said modulator ofsuch frequency and plit that when combined with waves cs d freomponentthe output comprising a balanced third order modulator having twoconjugately connected inputs, means for impressing on one input of saidmodulator a wave of the frequency f, a circuit including a tunedamplifier coupling the output of said modulator to the other inputthereof, the tuning and gain of said amplifier being made such that thecoupling circuit selectively feeds back to said other input a wave ofthe sub-multiple frequency in suflicient amount so that the combinationproducts in the output of said modulator include a sustained componentof the frequency and means to select from the output of said amplifier awave of the frequency 3. A circuit for deriving from a wave of a basefrequency f a wave of the sub-multiple frequency comprising a balancedthird order modulator having two conjugately connected inputs, means forimpressing on one input of said modulator a wave of the base frequencyf, a circuit connecting the output of said modulator to the other inputthereof to feed back waves derived from said output, the feedbackcircuit comprising in order an amplifier connected to the output of saidmodulator tuned to selectively amplify the frequency a frequency doublerfor transforming the frequency 5 to the frequency and a second amplifierfor selectively amplifying and impressing on said other input of them0dulator the Wave of the frequency to modulate therein with theimpressed base frequency f, and means for picking off from the output ofthe first amplifier a sustained wave of the frequency 4. A circuit forproducing accurate odd submultiples of a given frequency, comprising abalanced third order modulator having two conjugately connected inputcircuits and an output circuit, means to impress a wave of said givenfrequency on one of said input circuits, a regenerative circuit couplingsaid output circuit to the other input circuit, tuned to selectivelyfeed back to said other input circuit waves of the desired oddsub-multiple frequency to combine with the waves of given frequency insaid modulator, and means to pick off from said regenerative circuitsustained waves of said desired odd sub-multiple frequency.

5. A circuit for producing waves which are accurate odd sub-multipleratios of a given frequency, comprising a balanced third order modulatorhaving two conjugately connected input circuits and an output circuit,means to impress a wave of said given frequency on one of said inputcircuits, a regenerative circuit coupling said output circuit to theother of said input circuits, means in said regenerative circuit forderiving from the waves impressed thereon from the output of saidmodulator, and transmitting to said other input circuit to modulate withthe impressed wave of given frequency in said modulator, an amplifiedwave of double the frequency of the desired sub-multiple frequency, andmeans for picking off from said regenerative circuit a sustained wave ofthe desired odd sub-multiple frequency.

6. A circuit for producing accurate odd fractional sub-multiples of agiven frequency comprising a balanced third order modulator having twoconjugately connected input circuits and an output circuit, means forimpressing a wave of W said given frequency on one of said inputcircuits, a regenerative circuit coupling said output circuit with theother of said input circuits, said regenerative circuit comprising inorder an arm plifier connected to the output of said modulator tuned toselectively amplify waves of the desired sub-multiple frequency, afrequency multiplier for producing a multiple of the selected frequencyand means for amplifying and impressing the multiplied frequency on saidother input circuit of said modulator to modulate therein with theimpressed wave of said given frequency, said multiple frequency producedby said multiplier and its amplification being selected to produce asone of the products of modulation a sustained component of the desiredsub-multiple frequency, and means for picking off from the output ofsaid tuned amplifier a sustained wave of said desired sub-multiplefrequency.

RALPH L. MILLER.

